Four-dimensional entanglement distribution over 100 km

Abstract

High-dimensional quantum entanglement can enrich the functionality of quantum information processing. For example, it can enhance the channel capacity for linear optic superdense coding and decrease the error rate threshold of quantum key distribution. Long-distance distribution of a high-dimensional entanglement is essential for such advanced quantum communications over a communications network. Here, we show a long-distance distribution of a four-dimensional entanglement. We employ time-bin entanglement, which is suitable for a fibre transmission, and implement scalable measurements for the high-dimensional entanglement using cascaded Mach-Zehnder interferometers. We observe that a pair of time-bin entangled photons has more than 1 bit of secure information capacity over 100 km. Our work constitutes an important step towards secure and dense quantum communications in a large Hilbert space.

Figure 1

Experimental setup. (a) Generation and distribution of four-dimensional time-bin entanglement. (b) Alice and Bob’s measurements. PPLN, periodically poled lithium niobate waveguide; BPF, band pass filter; PC, polarization controller; WDM, wavelength demultiplexing filter; DSF, dispersion shifted fibre spool; Auto PC, remote controllable polarization controller; Pol, polariser; MZI, Mach-Zehnder interferometers; SNSPD, superconducting nanowire single photon detector. The inset shows the detection efficiencies of the SNSPDs.

Figure 2

Result of photon detection time tracking. Histograms of single photon counts in long-time measurements at SNSPD 2 for Alice are shown. Single photon counts were accumulated in one minute for each histogram. The photon detection time drifted about 4 ns during the measurement, which was longer than the total duration of the four-dimensional time-bin state.

Figure 3

Experimental results. (a) Real and (b) imaginary parts of the density operator reconstructed by QST. The data shown here were averaged over four trials. To increase readability, the local phase rotation $\hat{U}(\phi )={\sum }_k\exp (-ik\phi )|k{⟩}_s⟨k{|}_s$ was multiplied after averaging $\hat{\rho }$, where ϕ was calculated from the probability amplitudes of |00〉 and |11〉 in the eigenstate with the largest eigenvalue.